Problem: Solve for $k$ : $4 = -9 + k$
Solution: Add $9$ to both sides: $ \begin{eqnarray} \\ \hphantom{4 {+ 9}} &\hphantom{=}& \hphantom{k} \\ 4 &=& -9 + k \\ \\ {+9} && {+9} \\ \end{eqnarray} $ $ \begin{eqnarray} \\ 4 {+ 9} &=& k \\ \end{eqnarray} $ Simplify. $ k = 13$